Hi, Welcome to JustAnswer.com, I am glad you are here! My name is Eric & I will endeavor to answer your question.
What you are describing could be two things. Please try going over to the push button on the wall and pressing and holding it in. If the door closes while holding the button pressed, you are likely having one of the following issues with your safety sensors.
The solution is usually one or more of the following.
A. The sensors are dirty - Wipe lenses with a clean soft cloth
B. They are misaligned (This is the most likely problem and the most common issue with Garage Door openers). Follow the safety sensor alignment instructions in your owners manual.
C. There is a broken or disconnected wire between the motor and the safety sensors.
Carefully follow the wire path and do a visual inspection to ensure a continuous connection.
D. You have a defective sensor which requires replacement.
If pushing the wall button and holding it does not make the door fully close then it is quite possible that your door is reversing itself because it is sensing undue resistance. This is a built-in safety feature that is there in the event that something or someone gets in the way of the closing door. If it senses resistance before it reaches its downward limit switch, it reverses itself or stops. Often as a garage door ages, its moving parts don't slip and slide as freely as they did when the unit was new. The manufacturers had foresight enough to understand this and built in a way for the homeowner to make an adjustment and compensate for this added resistance.
Up on the motor are 2 dials. These are user accessible. They could be under the plastic light shield which you might have to remove in order to gain access to them. Sometimes these are just a shaft about 1/4" in diameter sticking out of the back with a slot for a screwdriver and sometimes you have to insert a small screwdriver into a hole. There should be some type of directional indicator telling you which is more and which is less and which is up and which is down.
These are upward and downward force adjustments. Increasing the downward force slightly will tell the system to push through a greater resistance before it reverses itself or stops.
Increasing the up force adjustment will do the same thins only in the up direction.
Try increasing the downward force just a little. If that does not solve the problem, try increasing it some more. If this does not solve your problem, please post back here and we will move on to the next step.